Speaking frankly(then I'm done)
I think I was exactly where you are now,3 mos. ago.0 experience,even with GBA flashcarts.
Count on spending some time getting it running.
It may be easier now than it was(updates are frequent),but you will get pissed,some.You will get happy,a lot.
Hardly any documentation,but plenty of help here.
The cheaper package has a 512mb(thats megabit-learn about that somewhere else here)MK3.this sticks out of the back of your DS,and can store,and run roms.
The MK3 has a slot on it for your SD card(available at any drugstore),which can store and run roms.you write to it on your P.C.
It also has a NEO-Key,which is a substitute to a GBA flashcart.It holds almost no info,but is necessary without a NEO-Flashcart(or any other brand)
The more expensive package has a smaller(128mb)MK3,but includes a 512mb GBA flashcart.
You can't set up for GBA and NDS at the same time(I dont think),but you have the option of either.I do not think you can run GBA(yet-please,someone correct me?)with the cheaper package.
Some roms are 1g(512 x 2).you can get a 1g flashcart,and soon to come is a MK3 1g,that you should be able to purchase seperately,but that's another can of worms.
Good luck,got some Nifty homebrew to check out.....
I know it is difficult to get my point sometimes(i ramble),but slow down,read this(and all over this forum)and you will understand